The Theodolite

A
theodolite is a precision instrument for measuring angles in the horizontal and
vertical planes. Theodolites are mainly used for surveying applications, and
have been adapted for specialized purposes in fields like meteorology and
rocket launch technology. A modern theodolite consists of a movable telescope
mounted within two perpendicular axes — the horizontal or trunnion axis, and
the vertical axis. When the telescope is pointed at a target object, the angle
of each of these axes can be measured with great precision, typically to
seconds of arc.

Transit
refers to a specialized type of theodolite developed in the early 19th century.
It featured a telescope that could "flip over" ("transit the
scope") to allow easy back-sighting and doubling of angles for error
reduction. Some transit instruments were capable of reading angles directly to
thirty seconds. In the middle of the 20th century, "transit" came to
refer to a simple form of theodolite with less precision, lacking features such
as scale magnification and micrometers. Although precise electronic theodolites
have become widespread tools, the transit still finds use as a lightweight tool
on construction sites. Furthermore, the Brunton Pocket Transit, commonly
employed for field measurements by geologists and archaeologists, has been in
continuous use since 1894. Some types of transits do not measure vertical
angles.